FIG. 1a shows a prior art gas discharge power supply 10 including a capacitor 12 which is charged by voltage source 15 in series with current limiting resistor 16. When the voltage level of capacitor 12 reaches a desired level, an ignition 14 is triggered, which acts as a switch device delivering charge from the storage capacitor 12 to a series combination of lead inductance 18, and a lamp assembly 21 which is electrically modeled as a gas discharge lamp 22, which acts as a constant voltage drop, in series with an arc resistance 20, which has a current-dependant voltage drop. Typically, the arc resistance 20 is very small compared to either the inductive impedance of lead inductance 18 or the capacitive reactance of storage capacitor 12, thereby producing an under-damped series RLC circuit. FIG. 2 shows the waveforms of operation of FIG. 1a. At a time t=0 us, ignition 14 is triggered and operates as a closed circuit, resulting in the transfer of energy from storage capacitor 12 to the series circuit of lamp assembly 21 including resistance 20, and lead inductance 18. The current which results from the ignition 14 switch closing is an oscillatory LRC decay I1 32 shown in FIG. 2, where frequency and decay are determined by L R and C according to the well-known formula:
      I    ⁡          (      t      )        =            I      max        ⁢          ⅇ                                    -            R                                2            ⁢            L                          ⁢        t              ⁢          sin      ⁡              (                                                            1                LC                            -                                                (                                      R                                          2                      ⁢                      L                                                        )                                2                                              ⁢          t                )            
When R=0.01 ohms, C=0.5 uF and L=50 nH in FIG. 1a, the current waveform I1 32 is oscillatory as shown in FIG. 2, and lamp 22 generates multiple bursts of optical energy 28, shown as waveform E1 30. Each burst of optical energy 28 is approximately 1 μs in duration, and multiple bursts are emitted until the oscillatory voltage which appears across the gas discharge lamp 22 falls to below the actuation level of the lamp 22. This results in a plurality of optical bursts at the rate of oscillatory decay, with each subsequent optical pulse of reduced magnitude compared to the previous burst.
In applications where the lamp 22 is generating an optical burst 28 for use as control energy for an UV/optical switch such as a diamond switch, or some other photo-conducting device using UV/optical control, and the optical energy level is often required to be large in magnitude and short in duration, a problem arises whereby the size of the capacitor C 12 (due to limits on the applied voltage V 15) becomes too large to support the burst energy requirement. This increased capacitance 12 causes the resonant frequency to be reduced, which increases the time duration and reduces the rise time of the optical control signal produced by the gas discharge lamp 22.
It is desired to reduce the duration of the oscillatory decay, and further to capture the energy associated with the oscillatory decay and redirect it to the optical lamp, thereby producing a single, uni-polar pulse of current, which translates into a single burst or pulse of emitted optical energy 28.
An alternative embodiment 21 of prior art FIG. 1a, shown in FIG. 1b, places a second closing switch 15 directly in parallel with both the capacitor 12 and switch 14, and the flash lamp assembly 21. The first switch 14 is closed at an initial time t1, followed at time t2 by second closing switch 15, where the first switch 14 closing time and second switch 15 closing time is controlled by controller 17, and the second switch 15 is triggered to close at the time of the first quarter period following the first switch 14 closure. This method also has the disadvantage that for some circuit parameters, the current through the gas discharge lamp can reverse direction, thereby allowing the current to pass through zero and allowing the lamp discharge gas to begin cooling, which results in reduced optical emission from the lamp.
U.S. Pat. No. 3,465,203 by Galster et al describes a circuit for discharging stored charge into a flashlamp using inductors, capacitors, and diodes. Resonant current from the inductor/capacitor combination is redirected through clamping diodes to extend the capacitor discharge time.
U.S. Pat. No. 4,194,143 by Farkas et al describes the use of a resonant LC circuit to generate multiple flash lamp discharges.
U.S. Pat. No. 4,524,289 by Hammond et al describes a flash lamp using inductors, capacitors, and switches to transfer current from two resonant LC circuits to a flash lamp load.
A flash lamp control circuit is desired which generates a single pulse of current which can be optimized for power output and minimized for time duration.